Field of the Invention
The present invention relates to a actuation module, and in particular to a valve actuation module for actuating valve elements, such as a gate valve slab, needle stem, ball valve sphere or a choke cage or plug, on a subsea wellhead, but can equally be applied to other devices that require operating in a subsea environment.
Currently perceived wisdom is that subsea wellheads should have hydraulically operated valves which are provided with a fail-safe mechanism in the form of a large spring arranged to push the valve element into a safe position upon loss of hydraulic pressure. Generally speaking, it is possible to use the well pressure to assist in this movement of the valve element. However, for safety reasons it is still necessary to have a spring which is large enough that it will close the valve without assistance from the well pressure or in the reverse situation where external pressure is higher than well pressure.
With the current trend to deeper water wells, there is an increase in hydrostatic pressure but a possible decrease in well pressure. Thus, the net pressure differential can work against fail safe forces on the stem to close the valve. In this situation, the spring must be designed to be large enough to overcome the friction between the valve elements, the housing seals, and to overcome the hydraulic forces to expel the hydraulic control fluid to close the valve against the hydrostatic pressure and low internal well pressure. It can be readily appreciated from this that at increasing water depths, the size of the spring must be greatly increased thereby increasing the size and cost of the system. Further, the size of the force delivered by the spring has an adverse effect on the high power required for each operation and on its sensitivity.
An alternative approach is disclosed, for example, in U.S. Pat. Nos. 4,920,811 and 5,195,721 in which the valve elements are electrically operated by powerful electrical motors using considerable electrical power. This would require immense storage capacity if not directly supplied by the umbilical from the remote installation. In both cases, a clutch is provided to disconnect the valve element in the event of a loss of power, thereby allowing the spring to move the valve into a fail-safe position. In this case, although it is no longer necessary for the spring to overcome hydraulic control fluid pressure being electric, a large spring is required to overcome the valve friction and the hydrostatic pressure. Furthermore, the clutch mechanism needed to disengage the valve element results in an unnecessarily costly and complicated structure.
The inventor has appreciated that the above problems can be overcome by the revolutionary new approach of operating the devices, not by a hydraulic actuator, with a fail safe return spring, but by means of a rotary actuator driven by an electric motor which is normally energised from an external source through an umbilical. The necessary fail safe condition is achieved by providing the motor with a local backup power source which, in the event of loss of primary power, is sufficient to drive the motor to return the device to a safe position.